The dearth of reliable data to permit estimation of intra-subject variability in male hormones over time (both short and longer-term) is a glaring deficiency in the field of male reproductive endocrinology. Data from our Massachusetts Male Aging Study (MMAS) suggests that hypogonadism may not be a stable syndrome over time. Moreover, data on intra-subject variability is essential to the design of any scientifically credible trial of androgen replacement in men. These data are also important at the level of patient care- especially the adequacy of a single early morning serum sample for diagnostic purposes. This study has three primary aims: (1) To obtain reliable estimates of short term (day-to-day) and longer-term (six months) intra-subject variation in hormone levels in adult men. While the main focus is on testosterone (total, free, bioavailable and the free androgen index), other major hormones (DHT, SHBG, LH, prolactin, DHEA, DHEAS, cortisol, androstenedione, androstanediol glucoronide (3-alpha-diol G) and estrone) are included because of rapidly growing interest in their actions, and because it's cost-efficient also to gather these data in a single study; (2) To determine if the intra-subject variation in hormone levels changes with age (a few studies have focused on relatively young men); (3)To examine the magnitude of any seasonal variability (this could affect clinical interpretation of results and power calculations in clinical trials). It is also important to estimate the relative contribution of assay variation and biological variation to total intra-subject variation. The proposed research builds on nearly 20 years of work on the MMAS and results will have an impact at several levels- the epidemiology of male endocrine change, clinical trials design and the clinical management of individual patients with androgen deficiency.